3 replies to this topic

[lyok]

Hallo all,

I have a question regarding selective media.

I have been reading on a medium that they use to identify  bacteria.
One of the statements they make is that the specific bacterium that you look for can be seen as a yellow bacterium since they can use the sucrose as a c-source and they will turn yellow because of this.

But why would they change from the original color of the medium into yellow?

I suppose they add some pH indicator in the medium too that will change color when the sucrose will be fermented and the pH drops?

or ?

[maryjo]

lyok, on Apr 11 2010, 09:10 AM, said:

Hallo all,

I have a question regarding selective media.

I have been reading on a medium that they use to identify  bacteria.
One of the statements they make is that the specific bacterium that you look for can be seen as a yellow bacterium since they can use the sucrose as a c-source and they will turn yellow because of this.

But why would they change from the original color of the medium into yellow?

I suppose they add some pH indicator in the medium too that will change color when the sucrose will be fermented and the pH drops?

or ?

Ya thats right. Its a pH indicator that has one color in acidic medium and another in a basic medium.

[josse]

Eum intersting topic, I have however seen the following question somewhere else and wonder what you guys think about it.


Will the pH drop then or can it go up too?

I can imagine it will only go down since you want to select by fermentation of a typical sugar or substance.


or?

Edited by josse, 16 April 2010 - 05:06 AM.

[pito]

You can have both: pH drop or a rise.

It depends on the medium and what you look for.

The use of peptone is an example. This can make the pH go up.